Automotive engine oils typically comprise a lubricant base stock and an additive package, both of which can contribute significantly to the properties and performance of the automotive engine oil.
The choice of lubricant base stock can have a major impact on properties such as oxidation and thermal stability, volatility, low temperature fluidity, solvency of additives, contaminants and degradation products, and traction. The American Petroleum Institute (API) currently defines six groups of lubricant base stocks (API Publication 1509).
Groups I, II and III are mineral oils which are classified by the amount of saturates and sulphur they contain and by their viscosity indices.
Table One below illustrates these API classifications for Groups I, II and III.
TABLE ONEGroupSaturatesSulphurViscosity IndexI<90%>0.03%80-120IIAt least 90%Not more than 0.03%80-120IIIAt least 90%Not more than 0.03%At least 120
Group I base stocks are solvent refined mineral oils, which are the least expensive base stock to produce, and currently account for the majority of base stock sales. They provide satisfactory oxidation stability, volatility, low temperature performance and traction properties and have very good solvency for additives and contaminants. Group II base stocks are mostly hydroprocessed mineral oils, which typically provide improved volatility and oxidation stability as compared to Group I base stocks. They also respond differently to additives as compared to Group I base stocks. The use of Group II stocks has grown to about 30% of the US market. Group III base stocks are severely hydroprocessed mineral oils or they can be produced via wax or paraffin isomerisation. They are known to have better oxidation stability and volatility than Group I and II base stocks but have a limited range of commercially available viscosities.
Group IV base stocks differ from Groups I to III in that they are synthetic base stocks i.e. polyalphaolefins (PAOs). PAOs have good oxidative stability, volatility and low pour points. Disadvantages include moderate solubility of polar additives, for example antiwear additives
Group VI base stocks are polyinternalolefins (PIOs), which are of a similar chemistry to PAOs in that both are manufactured by the oligomerisation of linear olefins. They offer similar performance in engine tests to Group III and Group IV base stocks and are slightly inferior to PAOs in terms of viscosity index, volatility, and low temperature properties.
Group V base stocks are all base stocks that are not included in the other Groups. Examples include alkyl naphthalenes, alkyl aromatics, vegetable oils, esters (polyol esters, diesters and monoesters), polycarbonates, silicone oils and polyalkylene glycols.
To create a suitable engine oil, additives are blended into the chosen base stock. The additives either enhance the stability of the lubricant base stock or provide additional protection to the engine. Examples of engine oil additives include antioxidants, antiwear agents, detergents, dispersants, viscosity index improvers, defoamers and pour point depressants, friction reducing additives.
One area of concern for automotive engines is around reduction of fuel consumption and energy efficiency. It is well known that the automotive engine oil has a significant part to play in the overall energy consumption of automotive engines. Automotive engines can be thought of as consisting of three discreet but connected mechanical assemblies which together make up the engine, the valve train, the piston assembly, and the bearings. Energy losses in mechanical components can be analysed according to the nature of the friction regime after the well known Stribeck curve. Predominant losses in the valve train are boundary and elastohydrodynamic, in the bearings are hydrodynamic, and the pistons hydrodynamic and boundary. Hydrodynamic losses have been gradually improved by the reduction of automotive engine oil viscosity (e.g. 5W30 instead of 10W30 viscosity grades). Elastohydrodynamic losses can be improved by careful selection of the base stock type, taking into account the traction coefficient of the base stock. Boundary losses can be improved by careful selection of friction reducing additive. Careful selection of both base stock and friction reducing additive is therefore important, but it is not as simple as choosing the best base stock for hydrodynamic and elastohydrodynamic properties, and then choosing a friction reducing additive which is known to be active in the boundary regime. The interaction of base stock, friction reducing additive and other additives needs to be considered.
Friction reducing additives that have been used to improve fuel economy fall into three main chemically-defined categories, which are organic, metal organic and oil insoluble. The organic friction-reducing additives themselves fall within four main categories which are carboxylic acids or their derivatives, which includes partial esters, nitrogen-containing compounds such as amides, imides, amines and their derivatives, phosphoric or phosphonic acid derivatives and organic polymers. In current commercial practice examples of friction reducing additives are glycerol monooleate and oleylamide, which are both derived from unsaturated fatty acids.
While initial fuel economy requirements, for which the above friction reducing additives were designed, focused only on the fresh oil, new engine oil specifications have now been developed that also address fuel economy longevity. A good example is Sequence VI-B, an engine test, which has been developed for the ILSAC GF-3 specification. For the ILSAC GF-4 specification, Sequence VI-B includes ageing stages of 16 and 80 hours in order to determine fuel economy longevity as well as fuel economy. These ageing stages are equivalent to 4000-6000 miles of mileage accumulation required prior to the EPA Metro/Highway Fuel economy test. That test is used in determining the Corporate Average Fuel Economy (CAFE) regulation parameter for a vehicle.
WO03/031543 discloses a range of saturated friction reducing additives chosen from saturated amides themselves and in combination with saturated polyol esters, to address these fuel economy longevity requirements alongside the fuel economy requirements. However there is no disclosure of any optimising of the combination of these saturated friction reducing additives with base stock. There is only a general disclosure of choice of base stocks.
Accordingly, there remains a need for a lubricant composition for automotive engine oils comprising a combination of a specifically chosen base stock and a specifically chosen friction reducing additive designed to meet the fuel economy and fuel economy longevity requirements.